Does he go alone to this field when you're at home? If so, has he ever had "something come up" over there--got hurt, got into an argument, etc.--and how did he handle it? Does he have his own key to your house, and has he been responsible about keeping track of it? Was he very obedient to the rules last summer? The answers to these questions should help you decide whether he's old enough to go over to the field when you're not home.
If you decide to let him, stick to your rule about no friends over--he may be tempted to bring them home. Also stick to your rule about calling before he goes out (and when he comes back) so that you'll know why he's not answering if you call. Talk about what to do if he gets hurt and what to do if he loses his key.
I think it's fine for a 13yo in general to do this kind of thing, but you have to decide whether YOUR son is ready for it.
If you decide to let him, stick to your rule about no friends over--he may be tempted to bring them home. Also stick to your rule about calling before he goes out (and when he comes back) so that you'll know why he's not answering if you call. Talk about what to do if he gets hurt and what to do if he loses his key.
I think it's fine for a 13yo in general to do this kind of thing, but you have to decide whether YOUR son is ready for it.